This invention relates to bus structure comprising an insulating support and an electric bus bar mounted on the support and having all of its external surfaces in the region of the support covered with an unbroken insulating coating.
To permit a reduction in the electrical clearances between adjacent high voltage bus bars and also between the bus bars and ground, it is now common practice in certain applications, e.g., metal-clad switchgear rated at 13.8 kV, to provide the bus bars with a thin unbroken coating of insulation. While such an unbroken coating does permit some reductions in electrical clearance, the full reductions safely attainable with such unbroken insulating coatings typically have not been realized primarily because of the way in which the bus bars have been supported.
An insulating support that has permitted considerable reductions in electrical clearance as compared to those needed with a conventional porcelain insulator having metal end fittings is disclosed and claimed in U.S. Pat. No. 3,519,733-Eichelberger et al, assigned to the assignee of the present invention. This insulating support arrangement achieves this result primarily by providing attaching structure between the bus bar and the insulator that is free of metal parts and is effective without perforating the insulating coating.
While this general approach is quite effective and is one which we employ in our insulating support, we seek to obtain still further reductions in the required electrical clearances as compared to those needed with the Eichelberger et al and similar insulating structures. Our studies of this matter have shown that one obstacle to achieving such further reductions in required clearances is the ionization of critical air gaps between the coated conductor and the insulating support where they are in contact. Such ionization can trigger a damaging flashover of the insulating support. Unfortunately, it is extremely difficult, if not practically impossible, to eliminate all critical air gaps at the interface of the bus bar and the insulating support.